Control system

ABSTRACT

A control system for a rubber calender incorporates features that improve profile and machine direction thickness control. In one embodiment a single gauge scans continuously across the rubber sheet and is coupled to a first profile average computer (PAC) to average the thickness of one edge zone. While the gauge is scanning the first zone the gauge is also coupled to a second PAC that averages the thickness of a center zone including contiguous portions of the edge zones. The first PAC is used to average the other edge zone. Each averaged edge zone includes substantially the entire half of the sheet width, so that the average of the edge zones is substantially the average of the sheet width and more representative of the portions controlled by adjustment of the respective screwdown motors. The control actions are initiated at points in the operation that decrease the scan cycle duration, and make possible more frequent control actions.

United States Patent 3,413,192 11/1968 Beecher.... 162/259 [72] InventorRobertE.McCall XJX one HUN /3/ 525 3 3 2 2 a n m m own BGB 9 9 676 999NH 020 505 8000 121 767 333 m o m M 1 wo m mw C7 0 N d Wm AH 11 2 22Primary ExaminerMalcolm A. Morrison Assistant Examiner-- Edward .1. WiseAttorneys-William T. Fryer, I11 and C. Henry Peterson ABSTRACT: Acontrol system for a rubber calender incor- 235/1513, porates featuresthat improve profile and machine direction 235/l5l-35 thickness control.In one embodiment a single gauge scans l 21/00 continuously across therubber sheet and is coupled to a first [50] Field profile averagecomputer (PAC) to average the thickness of [45] Patented Dec. 7, 1971[73] Assignee Industrial Nucleonics Corporation [54] CONTROL SYSTEM 6Claims, 4 Drawing Figs.

[51] Int. .235/151.35,

one edge zone. While the gauge is scanning the first zone the gauge isalso coupled to a second PAC that averages the References Citedthickness of a center zone including contiguous portions of the edgezones. The first PAC is used to average the other edge [62/259 zone.Each averaged edge zone includes substantially the en- 72/234 X tirehalf of the sheet width, so that the average of the edge zones issubstantially the average of the sheet width and more representative ofthe portions controlled by adjustment of the S T N E T A m.l P mma Mme WTd m m m Mxmk hmk A H mmmm U999. NH 900 11 852 325 420 067 000 000 333.Iiliitlllillii TRAVERSING respective screwdown motors. The controlactions are initiated at points in the operation that decrease the scancycle duration, and make possible more frequent control actions.

CONTROLLER INDICATOR 12/1962 Ziffer O15 6/1965 Hansenetal................. 206 4/1966 Bossen 676 4/1968Clement.......................

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Robert E. MLCa/l ATTORNEY CONTROL SYSTEM BACKGROUND OF THE INVENTION 1.FIELD OF THE INVENTION The present invention relates to control systemsfor industrial processes that include a regulator apparatus to produceor manipulate sheet materials, such as a rubber calender control system.

2. DESCRIPTION OF THE PRIOR ART Numerous systems have been utilized tocontrol an industrial process. These systems have attempted to maintainone or more properties of the material produced or manipulated at apredetermined value, usually referred to as a target.

The complexity of a control system for sheet materials is due to thefact that a large area and quantity of material are being produced. Theindustrial process usually has several controls that may needadjustment. The measurements, based on which the adjustments are made,must be representative of the controlled property. Significantimprovement in sheet material quality and reduction in excess materialused can occur if a control system has accurate measurements andsufficient control actions.

An example of an industrial process that requires an extensive controlsystem is the rubber calender that produces tire fabric. The calenderessentially comprises rolls that fonn the rubber sheet by applying rawrubber to both sides of a cord mat. These are several adjustmentsavailable to make the sheet uniform in thickness across the sheet(profile) and along the sheet length (machine direction). Theseadjustments include separately changing the spacing between the rolls ateach end, or changing the relative spacing between the roll ends and acentral portion, usually by a roll axis movement (cross-axis adjustment)or by bending one roll in the center (roll bending).

The control systems for the rubber calender have included gauges thatcan be manually or automatically scanned across the sheet to measure theaverage thickness and initiate a control action that uniformly changesthe thickness in the machine direction. This adjustment was not enough,normally to maintain the desired target(s) in the profile direction andthe sheet could become too thick in the center or wedge shaped.

There have been several control systems for industrial processes thatattempted better control of the profile. Examples are found in U.S. Pat.No. 2,909,660, issued Oct. 20, 1959, U.S. Pat. No. 3,000,438, issuedSept. [9, I961, U.S. Pat. No. 3,006,225, issued Oct. 31, 1961, andpending U.S. Pat. application Ser. No. 75 l ,013, filed May 15, 1968.Each of the above patents and application is assigned to the assignee ofthe present application.

One of the most common control systems for a rubber calender used asingle gauge that continuously scanned across the sheet, separatelyaveraging successive zones. An edge zone average measurement was used tocontrol the respective edge roll spacing, and the opposite edge zoneaverage measurement wasused to control the respective edge roll spacing.The average measurement in a zone in the center was used to control thecross-axis or similar adjustment. The timing sequence of one form ofcontrol system with a single gauge to obtain successive zone averages isillustrated in FIG. 1. A single-profile average computer (PAC) is usedto average zone thickness. The PAC is cleared (erased) and zone 1 ismeasured for 8 seconds. The PAC average is read out, stored, and thenthe PAC is erased (2 seconds for the readout, storage and erasefunctions). The gauge moves continuously and the center zone is measuredfor 8 seconds. The center zone average is read out, stored, and the PACerased, again in 2 seconds, and edge zone 2 is measured. The readout,storage and control action for edge zone 2 occurs in 9 seconds. Thegauge then returns to its starting point at the opposite edge, taking 6seconds with a fast scan. The total time for a cycle of FIG. 1 (gaugescans over and back) is 43 seconds.

From this example several facts on the control system cycle of FIG. 1are apparent: (l) the edge zones are each averaged only 8 seconds; (2)not all the edge zone that is controlled by the roll end spacing isaveraged. The center zone measurement starts before the first half ofthe sheet is averaged and ends while the gauge is scanning the secondhalf of the sheet.

An 8 second averaging time may not be sufficient to reduce the efiect ofshort term machine direction variations that cannot be controlled. Thepresence of short term machine direction variations may require areduction in controller gain to reduce overcontrol, with some loss incontrol effectiveness. The failure to include one-half of the sheetwidth for each edge zone average makes the average less representative.

The total cycle time of 43 seconds means that a control action can bemade on each edge only once every 43 seconds.

SUMMARY OF THEINVENTION The control systems of the prior art function toimprove the industrial process. Certain additional improvements can bemade by embodying the features of the present invention. Morerepresentative profile measurements can be obtained. Better profilecontrol of the edge zones and central portion is possible. A greaterfrequency of control actions can occur.

The advantages can be achieved in accordance with the present invention.To improve the measurement representation of the edge zone average tothe true sheet average, the sheet is divided at the center andsubstantially all of one-half of the sheet is scanned to produce each ofthe edge zone averages. As a result, the average of the two edge zoneswill always equal substantially the total sheet width average. Thecenter zone overlaps the edge zones, and thereby does not reduce thewidth available to scan for the edge or center zones. A separate PAC canbe used to develop the center zone average independent of profile shape.The increased width of the edge zone increases the zone measuring timewhich is of benefit in obtaining better machine direction representationby reducing the effect of short term machine direction variations.

A shortened cycle to complete the zone averaging and control actions isaccomplished by making the control action for the second edge zone whilethe gauge is returning to the start scan point. The control action forthe central portion can be initiated immediately after the center zoneaverage is obtained. In the neighborhood of 20-30 percent more controlactions can be made.

Each of these improvements and the combination results in a controlsystem that has significant advantage over prior art control systems inmany industrial process applications.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a time-function chart showingthe sequence and duration of operations performed by an example of aprior art control system.

FIG. 2 is a time-function chart showing the sequence and duration ofoperations performed by one embodiment of a control system incorporatingfeatures of the present invention.

FIG. 3 is a schematic presentation of the sheet (plan view) showing theprofile zones and illustrating where along the scanning path certainfunctions are performed.

FIG. 4 is a schematic diagram of a rubber calender control systemarranged in accordance with one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) The illustration of FIG. Ican now be compared with FIG. 2, to introduce a preferred embodiment ofthe present invention. FIG. 2 shows the gauge scanning cycle beginningat the edge of zone 1.

Zones 1 and 2 are scanned for 10 seconds each, to scan substantiallyone-half the sheet width for each zone. Experience indicates that 8 to10 seconds is the minimum averaging time in order to reduce the effectof short term machine direction variations on the control effectiveness.Before the averaging of edge zone 1 is completed, the averaging of thecenter zone begins, and continues for 8 seconds, overlapping edge zones1 and 2. When edge zone 1 averaging ends, its average is read out andstored (2 seconds). The control action for zone 1 begins after the zone1 average is stored. The control action for the center zone can commenceafter its average is read out and stored (total 7 seconds).Alternatively, several different comparison techniques may be used, suchas the one to be mentioned in relation to FIG. 4, to insure a desiredrelation between the edge zones 1 and 2 and/or the center zone beforethe central portion control action occurs. In such a case, the first orsubsequent scans across the sheet may omit the central portion controlaction until the relation between the desired edge zones and/or centerzone has been established, and then the central portion control actioncan occur after the center zone averaging, on each forward scan, if therequired relationship exists. After the zone 2 average is read out andstored (2 seconds), the gauge moves back to the start position, with afast return. During the fast return the control of zone 2 can occur (7seconds to return scan and control). The total time for the cycle is 3 1seconds.

The edge zones 1 and 2 and the center zone 3 are illustrated in relationto rubber sheet 4 in FIG. 3. Sheet 3 moves in the direction of arrow 5.Edge zone 1 is the left-half of sheet 4 and edge zone 2 is theright-half. It is edge zones 1 and 2 that are separately controlled bythe rubber calender. Consequently, it is desirable to average as much ofedge zones 1 and 2 to develop a truly representative control signal foreach edge zone. Center zone 3 overlaps edge zones 1 and 2, being made upof contiguous portions of edge zones 1 and 2, to include the centralportion that is controlled by the rubber calender crossaxis orequivalent control in relation to the edge zones. Normally, the controlof the central portion is not initiated until the edge zones aresubstantially on target. A separate target can be used for the centerzone, or a method described in the above referenced patent applicationcan be used, comparing one edge average with the center zone average toinitiate a central portion correction only if the two averages are notequal, to restore their equality.

FIG. 3 shows in schematic form the areas where a single gauge 23traversing the width of sheet 4 will be in relation to other operationsof a control system 9 of FIG. 4. The measuring head 40 of gauge 23 has atransducer for measuring a material property, in this instance thethickness of the rubber in a small area. Several types of gauges can beused, such as shown in U.S. Pat. No. 2,790,945, issued Apr. 30, 1957,assigned to the assignee of the present application. Gauge 23 is movedacross sheet 4 by traversing means 6, at right angles to the sheetmovement direction. While gauge 23 actually traces out a diagonal lineacross sheet 4, for simplicity FIG. 3 shows the zones and areas ofoperation as if sheet 4 was not moving.

The property measuring signal from gauge 23 is processed in aconventional measuring circuit 7, coupled to profile average computer(PAC) 8 and PAC 11. The operational sequencing of control system 9 isthe function of programmer 10. PAC 8 is coupled to an indicator 12 toindicate the zone average, if desired, while PAC 11 is coupled toindicator 13 for the same purpose. PAC 8 is coupled also to aninformation storage device 14 that can retain a zone average forsubsequent use in a control action, while PAC 8 is erased. PAC 1] iscoupled also to a storage device 15 that can retain the center zoneaverage for subsequent use in a control action, while PAC 11 is erased.The use of separate storage devices is for the purpose of illustration,recognizing that a PAC could serve that function for some time beforebeing erased. The

retained zone averages in storage devices 14 and 15 can be compared bycomparator 16, to indicate a difference therebetween. If there is adifference, a signal proportional to the degree is coupled to switch 17that also receives the separate outputs of storage devices 14 and 15.The appropriate input to switch 17 is coupled to controller 18 thatincludes switch means (not shown) arranged to activate one of thecalender adjustments: screwdown motor 28 for edge zone 1, screwdownmotor 30 for edge zone 2. or cross-axis adjustment motor 32 for thecenter zone 3. The detail operation of such a calender with cross-axisadjustment is well known, reference being made to the above-mentionedU.S. Pat. No. 3,006,225. In general, the calender includes a frame 27;upper roll 29 that has its axis movable in a vertical plane, to changethe spacing with respect to lower roll 25 for the edge zone adjustments.Lower roll 25 has its axis adjustable in a horizontal plane, pivotingabout a central point, to change the relationship between the roll edgesand a central portion, and thereby readjust the sheet profile.Cross-axis motor 32 uniformly drives each side of lower roll 25 inopposite directions, through drive shaft 31 to effect the cross-axismovement. A roll bending or other similar mechanism can be used instead,to change the relation between the edges and the central portion ofsheet 5. Controller 18 is merely representative, for separatecontrollers of one or more different types may be used for eachadjustment.

The sequence of operation of control system 9 can be explained byreferring to FIGS. 3 and 4. Gauge 23 is initially at one each of sheet4, adjacent edge zone 1. Gauge 23 may have just completed its automaticwithdrawal standardization function, as described in the aforementionedU.S. Pat. No. 2,790,945. Programmer 10 initiates the scanning cycle,diagrammed in FIG. 2, through traversing means 6. Gauge 23 scans at aconstant speed across sheet 5, to the opposite edge 15 and returns toits start position at the edge zone 1. In some applications the gaugemay dwell adjacent one or both edges, although such an arrangement isnot illustrated here. PAC 8 starts averaging the measuring signal fromgauge 23 when measuring heat 40 is at the edge of sheet 5. The sheetedge and other positions across sheet 5 can be indicated by a positionbridge in programmer 10 set up to activate a relay when a desiredposition is to be reached or by a switch on the gauge bracket at theappropriate position, such as illustrated by switch 60 at the center ofsheet 4. PAC 8 continues to average the gauge measurement for 10 secondsalmost to the center of sheet 4, as represented by centerline 45, usingsubstantially half the sheet width for the average of edge zone 1.Substantially before measuring head 40 reaches centerline 45, programmerl0 initiates PAC 11 averaging the measuring signal at point 50.Programmer 10 stops PAC 8 averaging at point 51, so that the average ofedge zone 1 is between the edge and point 51, as represented by line 52Programmer 10 causes the average obtained for zone 1 to be transferredto storage device 14 and to indicator 12. Then, programmer l0 erases theaverage from PAC 8 and PAC 8 is ready to start a new average of themeasurement signal beginning at point 61. Between points 51 and 61 atime dura tion of 2 seconds occurs while the readout, storage and eraseoperations take place, so that substantially all of each sheet half isaveraged for its respective edge zone. After point 61 and to theopposite edge, the stored average of zone 1 is coupled by programmer 10to screwdown motor 28, to initiate a control action if the average isoff target. A separate PAC can be used for each edge zone, to averagethe entire sheet width half.

The center zone averaging stops at point 53 when programmer 10deactivates PAC 11. The center zone average is transferred to storagedevice 15, PAC 11 is erased, and the central portion control action canbe initiated (the total time for transfer, erase and control action is 7seconds). Preferably the central portion control action does not occuron the first scan across. Programmer 10 does not activate switch 17 orcontroller 18 to initiate the center portion control action at leastuntil the next scan cycle.

The averaging of zone 2 by PAC 8 begins at point 61 and continues to theopposite edge, over the distance represented by line 54, substantiallyhalf of the sheet width. The edge zone 2 average is transferred byprogrammer 10 to storage device 14, and PAC 8 is erased (transfer anderase operations take 2 seconds). The control action for edge zone 2occurs during the return scan across sheet 4, as represented by line 55.

The stored averages for zones 1 and 2 are compared with selectedtarget(s) in controller 18, by programmer coupling the respective storedaverage through switch 17 and connecting the appropriate target(s) incontroller 18. If the edge zones are on target, no control actionoccurs. If either one of the edge zone averages is off target a controlaction is initiated to return the edge zone to target value.

The next scan cycle starts with programmer 10 retaining information onwhether an edge zone control action occurred during the previous scancycle. If a control action occurred for either edge zone during theprevious scan cycle or for edge zone 1 during the current scan cycle, nocentral portion control action will occur during the new cycle. Thereason for this sequence is that central zone adjustment is notdesirable until the zone edges are on target. A detailed explanation ofthis technique can be found in the aforementioned U.S. Pat. No.3,006,225. Programmer 10 sets up the proper sequence depending on thecontrol operations. Another approach is not to initiate the centralportion control action unless during the previous scan cycle both edgezone averages were within a predetermined deadband (limits) and thecurrent edge zone 1 average is with the deadband. The same result couldbe achieved by comparing the previous scan edge zone averages with eachother, to see if they are in a predetermined deadband, before thecentral portion correction is initiated. Programmer 10 can coordinatethese functions.

The new average for zone 1 would be transferred to storage device 14,the last cycle averages for zones 1 and 2 having been erased at thebeginning of the new scan cycle. The average for edge zone 1 is comparedwith the new average for center zone 3, in comparator 16 between points53 and 56. If there is no difference, comparator 16 and a control actionis not initiated. If there is a difference, programmer 10 initiates,through switch 17 and controller 18, the necessary control action toreturn the equality. This control technique is described in furtherdetail in the above referenced patent application. The central portioncontrol action occurs during the scan cycle distance from point 56 tothe opposite edge, as represented by line 57. The cycle is completed asdescribed above and successive cycles continue in accordance with thesequence outlined above.

The overall improvement resulting from control system 9 can besummarized as follows:

1. A more representative average measurement for the edge zone, byaveraging substantially all of each sheet-half for the respective edgezone.

2. A greater averaging time for each edge zone, to reduce the effect ofshort term machine direction variations.

3. A shorter cycle of operation, including averaging and control actiontime, to permit more frequent control actions and, consequently, closeradjustment to target specifications.

The overlapping center zone permits the increased averaging time and theaveraging of substantially all of the sheet width-half. The separate PACfor the center zone average allows the edge zone averaging and thecenter zone averaging to proceed separately, conserving time andallowing for a control action on the central portion during theaveraging of the second edge zone. The use of the return time to controlthe edge zones, further reduces the cycle time.

OTHER EMBODIMENTS The above-described embodiment is merely an example ofa preferred embodiment. lt is pparent to one skilled in the art that thecontrol system can take a number of forms to carry out the broadfunction of the present invention. For example, the present inventioncan be utilized with several different industrial processes where asheet material is formed or manipulated. The property measured andcontrolled can take several forms, such as weight per unit area. Thetransducer would be selected to measure the desired property. Theindividual units of the FIG. 4 control system can be of various designs,to perform the basic function of the present invention. For example,

an analog or digital system processing and control technique can beemployed. A special purpose or general digital computer can beprogrammed to perform the functions of control system 9 in combinationwith a gauge. Also, individual features of the present invention can beused alone or in various subcombinations with significant advantage,while the disclosed preferred combination has the most significantoverall performance.

These other arrangements and changes in the disclosed embodiments ofapparatus are included within the scope of the present invention asdefined in the appended claims.

lclaim: 1. Apparatus for maintaining a desired property profile across asheet material passing through a regular apparatus in continuousfashion, said regulator apparatus having means including a firstactuator for adjusting said property at one end portion thereof, meansincluding a second actuator for adjusting said property at the oppositeend portion thereof, means including a third actuator for adjusting saidproperty between said end portions relative to the property at a centralportion of the material said apparatus comprising:

gauge means to be located on the output side of said regulator apparatusfor separately averaging said property in a first zone fromsubstantially one edge to substantially the center of said sheet,separately averaging said property in a second zone substantially fromthe center of said sheet to substantially the other edge of said sheet,and separately averaging said property in a center zone includingcontiguous portions of said first and second zones; and

control means for energizing said first actuator for said one zone inresponse to said first zone property average, energizing said secondactuator for said second edge zone in response to said second zoneproperty average, and energizing said third actuator for said centerzone in response to said center zone property average.

2. Apparatus, as described in claim 1, wherein said property average ofsaid first and second zones is substantially equal to said propertyaverage of the total width, independent of profile shape.

3. Apparatus, as described in claim 1, wherein said gauge meanscomprises a single gauge that scans across said sheet from one edge tothe other, first profile average computer means coupled to said singlegauge to average separately said first zone and said second zones, and asecond profile average computer means coupled to said single gauge thatseparately averages said center zone.

4. Apparatus, as described in claim 1, wherein said gauge meanscomprises a single gauge for measuring said property that performs ascanning cycle including traversing continuously across said sheet fromsaid one edge to said other edge, and returning back across said sheet;a first profile average computer means; second profile average computermeans; programmer means for controlling the averaging time in relationto the position of said gauge; said programmer means initiatingaveraging of said single gauge measured property by said first profileaverage computer substantially at said one edge and terminatingaveraging at substantially said sheet center; first information storagemeans; said programmer means initiating readout of said first zoneaverage immediately after terminating said averaging to store said firstzone average in said first information storage means; said programmermeans initiating averaging of said single gauge measured property bysaid second profile average computer means at a time when said singlegauge is in said first zone and terminating averaging after said singlegauge is in said second zone, to obtain a property average for saidcenter zone, second information storage means; said programmer meansinitiating readout of said center zone average immediately aftertermination of said center zone averaging to store said first zoneaverage in said second information storage means; said programmer meansafter storage of said first zone average, successively initiating eraseof said first zone average from said first profile average computer,averaging said property in said second averages.

6. Apparatus. as described in claim 4, wherein the energization of saidfirst actuator commences during the time said single-gauge scans saidsecond zone, the energization of said third actuator commences duringthe time said single gauge is scanning said second zone, and theenergization of said second actuator occurs during said back scan acrosssaid sheet.

1. Apparatus for maintaining a desired property profile across a sheetmaterial passing through a regular apparatus in continuous fashion, saidregulator apparatus having means including a first actuator foradjusting said property at one end portion thereof, means including asecond actuator for adjusting said property at the opposite end portionthereof, means including a third actuator for adjusting said propertybetween said end portions relative to the property at a central portionof the material said apparatus comprising: gauge means to be located onthe output side of said regulator apparatus for separately averagingsaid property in a first zone from substantially one edge tosubstantially the center of said sheet, separately averaging saidproperty in a second zone substantially from the center of said sheet tosubstantially the other edge of said sheet, and separately averagingsaid property in a center zone including contiguous portions of saidfirst and second zones; and control means for energizing said firstactuator for said one zone in response to said first zone propertyaverage, energizing said second actuator for said second edge zone inresponse to said second zone property average, and energizing said thirdactuator for said center zone in response to said center zone propertyaverage.
 2. Apparatus, as described in claim 1, wherein said propertyaverage of said first and second zones is substantially equal to saidproperty average of the total width, independent of profile shape. 3.Apparatus, as described in claim 1, wherein said gauge means comprises asingle gauge that scans across said sheet from one edge to the other,first profile average computer means coupled to said single gauge toaverage separately said first zone and said second zones, and a secondprofile average computer means coupled to said single gauge thatseparately averages said center zone.
 4. Apparatus, as described inclaim 1, wherein said gauge means comprises a single gauge for measuringsaid property that performs a scanning cycle including traversingcontinuously across said sheet from said one edge to said other edge,and returning back across said sheet; a first profile average computermeans; second profile average computer means; programmer means forcontrolling the averaging time in relation to the position of saidgauge; said programmer means initiating averaging of said single gaugemeasured property by said first profile average computer substantiallyat said one edge and terminating averaging at substantially said sheetcenter; first information storage means; said programmer meansinitiating readout of said first zone average immediately afterterminating said averaging to store said first zone average in saidfirst information storage means; said programmer means initiatingaveraging of said single gauge measured property by said second profileaverage computer means at a time when said single gauge is in said firstzone and terminating averaging after said single gauge is in said secondzone, to obtain a property average for said center zone, secondinformation storage means; said programmer means initiating readout ofsaid center zone average immediately after termination of said centerzone averaging to store said first zone average in said secondinformation storage means; said programmer means after storage of saidfirst zone average, successively initiating erase of said first zoneaverage from said first profile average computer, averaging saidproperty in said second zone, and terminating averaging substantial atsaid other sheet edge; said stored first zone average energizing saidfirst actuator, said stored center zone signal energizing said thirdactuator, and said first profile average computer average from saidsecond zone energizing said second activator.
 5. Apparatus, as describedin claim 4, wherein said stored first zone property average is comparedwith said stored center zone property average, to energize said thirdactuator only if there is a difference in the compared propertyaverages.
 6. Apparatus, as described in claim 4, wherein theenergization of said first actuator commences during the time saidsingle-gauge scans said second zone, the energization of said thirdactuator commences during the time said single gauge is scanning saidsecond zone, and the energization of said second actuator occurs duringsaid back scan across said sheet.